Silicon photonic components fabricated using a bulk substrate

ABSTRACT

Structures including a photodetector and methods of fabricating such structures. A substrate, which is composed of a semiconductor material, includes a first trench, a second trench, and a pillar of the semiconductor material that is laterally positioned between the first trench and the second trench. A first portion of a dielectric layer is located in the first trench and a second portion of the dielectric layer is located in the second trench. A waveguide core is coupled to the pillar at a top surface of the substrate.

BACKGROUND

The present invention relates to photonics chips and, more particularly,to structures including a waveguide core and methods of fabricating suchstructures.

Photonics chips are used in many applications and systems including, butnot limited to, data communication systems and data computation systems.A photonics chip integrates optical components, such as waveguides,optical switches, and bends, and electronic components, such asfield-effect transistors, into a unified platform. Among other factors,layout area, cost, and operational overhead may be reduced by theintegration of both types of components. Silicon optical components relyon the thick buried oxide layer of a silicon-on-insulator wafer toprovide a bottom cladding.

Improved structures including a waveguide core and methods offabricating such structures are needed.

SUMMARY

In an embodiment of the invention, a structure includes a substratecomposed of a semiconductor material. The substrate includes a firsttrench, a second trench, and a pillar of the semiconductor material thatis laterally positioned between the first trench and the second trench.The structure further includes a dielectric layer having a first portionin the first trench and a second portion in the second trench. Awaveguide core is coupled to the pillar at a top surface of thesubstrate.

In an embodiment of the invention, a method includes forming a firsttrench and a second trench in a substrate comprised of a semiconductormaterial. A pillar of the semiconductor material is laterally positionedbetween the first trench and the second trench. The method furtherincludes forming a dielectric layer having a first portion in the firsttrench and a second portion in the second trench, and forming a firstwaveguide core that is coupled to the pillar at a top surface of thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate various embodiments of theinvention and, together with a general description of the inventiongiven above and the detailed description of the embodiments given below,serve to explain the embodiments of the invention. In the drawings, likereference numerals refer to like features in the various views.

FIGS. 1-6 are cross-sectional views of a structure at successivefabrication stages of a processing method in accordance with embodimentsof the invention.

FIG. 5A is a top view of the structure of FIG. 5 in which FIG. 5 istaken generally along line 5-5.

FIGS. 7-9 are cross-sectional views of a structure at successivefabrication stages of a processing method in accordance with alternativeembodiments of the invention.

FIG. 10 is a cross-sectional view of a structure in accordance withalternative embodiments of the invention.

FIG. 10A is a top view of the structure of FIG. 10 in which FIG. 10 istaken generally along line 10-10.

FIG. 11 is a cross-sectional view of a structure in accordance withalternative embodiments of the invention.

FIG. 11A is a top view of the structure of FIG. 11 in which FIG. 11 istaken generally along line 11-11.

FIG. 12 is a cross-sectional view of a structure in accordance withalternative embodiments of the invention.

FIG. 12A is a top view of the structure of FIG. 12 in which FIG. 12 istaken generally along line 12-12.

DETAILED DESCRIPTION

With reference to FIG. 1 and in accordance with embodiments of theinvention, trenches 10 are formed in a substrate 12. To that end, ahardmask 14 is formed over a top surface 11 of the substrate 12 andpatterned using photolithography and etching processes to define an etchmask. The hardmask 14 may be composed of a dielectric material, such assilicon nitride. The substrate 12 may be, for example, a bulk substratecomposed of a single-crystal semiconductor material, such as a bulksingle-crystal silicon substrate. With the hardmask 14 present, anetching process, such as a reactive ion etching process, is used to formthe trenches 10. The trenches 10 extend into the substrate 12 torespective bottoms 17 at a given depth, d, relative to the top surface11. In an embodiment, the trenches 10 may extend to a depth of about 2.5microns to about 3 microns. Each trench 10 has sidewalls 16 that extendto a trench bottom 17, and the semiconductor material of the substrate12 borders the sidewalls 16 and trench bottom 17 of each trench 10.

Pillars 18 containing portions of the single-crystal semiconductormaterial of the substrate 12 are located laterally between adjacenttrenches 10. The pillars 18 extend from the top surface 11 of thesubstrate 12 to the same depth as the respective bottoms 17 of thetrenches 10. The trenches 10 have a width, w1, between the sidewalls 16,and the pillars 18 have a width, w2, that is significantly less than thewidth, w1, of the trenches 10. In an embodiment, the width, w1, may begreater than or equal to two (2) microns, and the width, w2, may beabout fifty (50) nanometers to about one (1) micron. In an embodiment,the pillars 18 may have an aspect ratio that is less than or equal tosixty (60).

With reference to FIG. 2 in which like reference numerals refer to likefeatures in FIG. 1 and at a subsequent fabrication stage, a dielectriclayer 20 is formed that includes respective portions that fill thetrenches 10. The dielectric layer 20 may be composed of silicon dioxidethat is deposited by chemical vapor deposition and then planarized withchemical-mechanical polishing.

With reference to FIG. 3 in which like reference numerals refer to likefeatures in FIG. 2 and at a subsequent fabrication stage, the hardmask14 is removed and the dielectric layer 20 in the trenches 10 is thenrecessed with one or more etching processes such that the top surface 11of the substrate 12 is exposed. In particular, the pillars 18 ofsemiconductor material between the dielectric-filled trenches 10 areexposed at the top surface 11 of the substrate 12. In an embodiment, thedielectric layer 20 and the pillars 18 may be coplanar at the topsurface 11 of the substrate 12. In an alternative embodiment, thedielectric layer 20 and the pillars 18 may be substantially coplanar atthe top surface 11 of the substrate 12.

With reference to FIG. 4 in which like reference numerals refer to likefeatures in FIG. 3 and at a subsequent fabrication stage, shallow trenchisolation regions 22 are formed in the substrate 12. The shallow trenchisolation regions 22 may be formed by etching trenches to a given depthinto the substrate 12 and depositing a dielectric material, such assilicon dioxide, in the trenches, followed by planarization withchemical-mechanical polishing. The shallow trench isolation regions 22surround an active region of the substrate 12 that may be used tosubsequently fabricate a field-effect transistor. The trenches 10penetrate to a greater depth into the substrate 12 than the trenchesused to form the shallow trench isolation regions 22. In an alternativeembodiment, the shallow trench isolation regions 22 may be formed byportions of the dielectric layer 20 instead of being separately formed.

A dielectric layer 24 is then deposited and patterned by lithography andetching processes to open a region over the pillars 18 anddielectric-filled trenches 10. The dielectric layer 24 may be composedof a dielectric material, such as silicon nitride.

With reference to FIGS. 5, 5A in which like reference numerals refer tolike features in FIG. 4 and at a subsequent fabrication stage, waveguidecores 26 are formed by epitaxial growth at the top surface 11 of thesubstrate 12 and, in particular, by epitaxial growth from the topsurface of the pillars 18. The waveguide cores 26 may be composed ofsingle-crystal semiconductor material, such as single-crystal silicon,grown by an epitaxial growth process. In an embodiment, the waveguidecores 26 may be formed by a selective epitaxial growth (SEG) process inwhich semiconductor material nucleates for epitaxial growth onsemiconductor surfaces such as the single-crystal substrate 12, but doesnot nucleate for growth from insulator surfaces, such as the dielectriclayer 24 or the dielectric layer 20. The pillars 18 provide the crystalstructure for the epitaxial growth of the waveguide cores 26.

The portions of the dielectric layer 20 in the trenches 10 are arrangedbelow the waveguide cores 26, and each waveguide core 26 overlaps at itssides with the portions of the dielectric layer 20. Because theepitaxial growth is characterized by a vertical growth component and alateral growth component, the waveguide cores 26 may be shaped as curvedportions of cylinders extending along respective longitudinal axes andhaving a width that is greater than the width of the pillars 18. In anembodiment, the width of the waveguide cores 26 may be equal toapproximately three times the width of the pillars 18, and the height ofthe waveguide cores 26 may be equal to approximately three times thewidth of the pillars 18.

With reference to FIG. 6 in which like reference numerals refer to likefeatures in FIG. 5 and at a subsequent fabrication stage, the dielectriclayer 24 is removed using an etching process. A field-effect transistor30 may be fabricated by front-end-of-line (FEOL) processing as a devicestructure in the active region of the substrate 12 surrounded by theshallow trench isolation regions 22. The field-effect transistor 30 mayinclude a gate electrode 32 and a gate dielectric formed by depositing alayer stack and patterning the layer stack with photolithography andetching processes. The gate electrode 32 may be composed of a conductor,such as doped polycrystalline silicon (i.e., polysilicon) or a workfunction metal, and the gate dielectric may be composed of an electricalinsulator, such as silicon dioxide or hafnium oxide. The field-effecttransistor 30 may include other elements such as halo regions, lightlydoped drain extensions, source/drain regions 34 in the well, andsidewall spacers on the gate electrode 32. In an alternative embodiment,a different type of transistor, such as a heterojunction bipolartransistor, may be fabricated instead of the field-effect transistor 30.

The portions of the dielectric layer 20 provide a bottom cladding forthe waveguide cores 26. The dielectric layer 20 replaces the buriedoxide layer of a silicon-on-insulator substrate, which enables the useof a cheaper bulk wafer as the substrate 12 for fabricating thephotonics chip.

With reference to FIG. 7 in which like reference numerals refer to likefeatures in FIG. 1 and in accordance with alternative embodiments, thetrenches 10 may be etched into the substrate 12 to a depth that is lessthan the full intended depth. The partially-etched trenches 10 may havea width that is slightly greater than w1 (FIG. 1). Sidewall spacers 36may be formed that line the partially-etched trenches 10 and hardmask14. The sidewall spacers 36 may be composed of a dielectric materialthat is deposited by a conformal deposition process, such as siliconnitride deposited by atomic layer deposition, and then etched with ananisotropic etching process, such as reactive ion etching.

With reference to FIG. 8 in which like reference numerals refer to likefeatures in FIG. 7 and at a subsequent fabrication stage, the trenches10 are further etched to their full given depth into the substrate 12,as described in connection with FIG. 1, after forming the sidewallspacers 36. A dielectric layer 38 is formed in the substrate 12 at thesidewalls 16 and bottom 17 of each trench 10. In an embodiment, athermal oxidation process may be used to form the dielectric layer 38.The formation of the dielectric layer 38 narrows the width of thepillars 18 as a thickness of the semiconductor material of the pillars18 is converted to silicon dioxide by the thermal oxidation process. Inan embodiment, the initial width of the pillars 18 may be increased suchthat a final width target (e.g., about 50 nanometers to about 1 micron)is achieved after performing the thermal oxidation process. In anembodiment, the pillars 18 may have the width, w2 (FIG. 1).

In an alternative embodiment, the semiconductor material of the pillars18 may be implanted with ions to damage the crystal structure of thesubstrate 12 along the sidewalls 16 of the trenches 10 and therebyincrease the oxidation rate. The implantation conditions (e.g., ionspecies (e.g., boron, arsenic, germanium, silicon, oxygen), dose,kinetic energy, tilt angle) may be selected to tune the amount of damageimparted to the crystal structure of the substrate 12.

With reference to FIG. 9 in which like reference numerals refer to likefeatures in FIG. 8 and at a subsequent fabrication stage, processingcontinues as described in connection with FIGS. 2-6 to form thewaveguide cores 26 and the field-effect transistor 30. The sidewallspacers 36 are present in the final structure beneath the top surface 11and under the waveguide cores 26. Each sidewall spacer 36 is positionedin a vertical direction between one of the waveguide cores 26 and one ofthe dielectric layers 38, and each sidewall spacer 36 is positioned in alateral direction between the portion of the dielectric layer 24 in oneof the trenches 10 and one of the pillars 18. Each sidewall spacer 36and its underlying dielectric layer 28 may have a summed height, h,(FIG. 8) relative to the top surface 11 that is equal or substantiallyequal to the depth of the trenches 10.

With reference to FIGS. 10, 10A in which like reference numerals referto like features in FIG. 6 and in accordance with alternativeembodiments, a photodetector 40 may be formed as a conformal layer overand adjacent to one of the waveguide cores 26. In an embodiment, thephotodetector 40 may be composed of a semiconductor material, such asgermanium or a germanium-tin alloy, that is epitaxially grown ordeposited over the waveguide core 26 and patterned with lithography andetching processes. Layers 42, 44 are formed over the portions of thedielectric layer 24 in the trenches 10. The layer 42 may be doped tohave n-type conductivity, and the layer 44 may be doped to have p-typeconductivity. The layers 42, 44, which may serve as contacts for thephotodetector 40, may be doped by ion implantation using a separateimplantation mask, or may be doped by co-implantation when thesource/drain regions 34 of the field-effect transistor 30 are formed.

With reference to FIGS. 11, 11A in which like reference numerals referto like features in FIG. 6 and in accordance with alternativeembodiments, the waveguide cores 26 may constitute the arms of aMach-Zehnder interferometer modulator. The arms of the Mach-Zehnderinterferometer modulator are coupled to an input waveguide 54 thatguides laser light to the arms and an output waveguide 56 that guidesmodulated laser light from the arms. A heater 50 is formed adjacent tothe waveguide core 26 that is its nearest neighbor. The heater 50 may bea silicided resistor embodied in a partially-silicided polysilicon bodythat includes a silicided upper section formed by a silicidationprocess. The heater 50 may be located over a shallow trench isolationregion 22, which provides thermal isolation during operation.

A layer 52 provides a bridge that connects the heater 50 with theadjacent waveguide core 26. The layer 52, which may be composed ofpolysilicon patterned by photolithography and etching processes,provides a path for the transfer of heat from the heater 50 to thewaveguide core 26. The layer 52 may overlap with a section of theadjacent waveguide core 26. The layer 52 may be located in part over theshallow trench isolation region 22 between the heater 50 and thewaveguide core 26, as well as in part over an adjacent portion of thedielectric layer 20. The dielectric materials thermally isolate theheater 50 from the substrate 12 during operation.

During operation, heat transferred to the waveguide core 26 from theheater 50 through the layer 52 may be used to modulate laser lightreceived from the input waveguide 54 to provide digitized output signalsat the output waveguide 56 by changing the permittivity (i.e.,dielectric constant) of the heated waveguide core 26. The change to thepermittivity of the heated waveguide core 26 prompts a change in therefractive index of its semiconductor material.

With reference to FIGS. 12, 12A in which like reference numerals referto like features in FIGS. 11, 11A and in accordance with alternativeembodiments, the waveguide cores 26 may be elements incorporated into adifferent type of Mach-Zehnder interferometer modulator. In that regard,a well 60 may be formed in the substrate 12 beneath the trenches 10. Thepillars 18 and lower portions 29 of the waveguide cores 26 may be dopedto have the same conductivity type as the well 60. A reach-throughregion 62 of the same conductivity type is formed adjacent to thewaveguide cores 26. The reach-through region 62 extends from the topsurface 11 of the substrate 12 to the well 60.

Upper portions 31 of the waveguide cores 26 may be doped to have anopposite conductivity type from the lower portions 29 of the waveguidecores 26, which defines p-n junctions 64. In an embodiment, the upperportions 31 of the waveguide cores 26 may be doped with low-energyimplantation.

Heavily-doped contacts 66 having the same conductivity type as the upperportions 31 of the waveguide cores 26 are formed on the dielectric layer20. The heavily-doped contacts 66 are respectively coupled to the upperportions 31 of the waveguide cores 26. A heavily-doped contact 68 isformed in the reach-through region 62 at the top surface 11 of thesubstrate 12. In an embodiment, the pillars 18, lower portions 29 of thewaveguide cores 26, well 60, reach-through region 62, and heavily-dopedcontact 68 may be doped to have n-type conductivity, and the upperportions 31 of the waveguide cores 26 and the heavily-doped contacts 66may be doped to have p-type conductivity.

The methods as described above are used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (e.g., as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. Thechip may be integrated with other chips, discrete circuit elements,and/or other signal processing devices as part of either an intermediateproduct or an end product. The end product can be any product thatincludes integrated circuit chips, such as computer products having acentral processor or smartphones.

References herein to terms modified by language of approximation, suchas “about”, “approximately”, and “substantially”, are not to be limitedto the precise value specified. The language of approximation maycorrespond to the precision of an instrument used to measure the valueand, unless otherwise dependent on the precision of the instrument, mayindicate +/−10% of the stated value(s).

References herein to terms such as “vertical”, “horizontal”, etc. aremade by way of example, and not by way of limitation, to establish aframe of reference. The term “horizontal” as used herein is defined as aplane parallel to a conventional plane of a semiconductor substrate,regardless of its actual three-dimensional spatial orientation. Theterms “vertical” and “normal” refer to a direction perpendicular to thehorizontal, as just defined. The term “lateral” refers to a directionwithin the horizontal plane.

A feature “connected” or “coupled” to or with another feature may bedirectly connected or coupled to or with the other feature or, instead,one or more intervening features may be present. A feature may be“directly connected” or “directly coupled” to or with another feature ifintervening features are absent. A feature may be “indirectly connected”or “indirectly coupled” to or with another feature if at least oneintervening feature is present. A feature “on” or “contacting” anotherfeature may be directly on or in direct contact with the other featureor, instead, one or more intervening features may be present. A featuremay be “directly on” or in “direct contact” with another feature ifintervening features are absent. A feature may be “indirectly on” or in“indirect contact” with another feature if at least one interveningfeature is present.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration but are not intended tobe exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A structure comprising: a bulk substratecomprised of a semiconductor material, the bulk substrate including atop surface, a first trench extending from the top surface into the bulksubstrate, a second trench extending from the top surface into the bulksubstrate, and a first pillar of the semiconductor material laterallypositioned between the first trench and the second trench; a firstdielectric layer having a first portion in the first trench and a secondportion in the second trench; and a first waveguide core directly on thefirst pillar at the top surface of the bulk substrate, the firstwaveguide core overlapping in part with the first portion of the firstdielectric layer at the top surface of the bulk substrate, and the firstwaveguide core overlapping in part with the second portion of the firstdielectric layer at the top surface of the bulk substrate, wherein thefirst portion and the second portion of the first dielectric layer arearranged below the first waveguide core.
 2. The structure of claim 1wherein the first pillar, the first portion of the first dielectriclayer, and the second portion of the first dielectric layer aresubstantially coplanar at the top surface of the bulk substrate.
 3. Thestructure of claim 1 wherein the first trench has a depth, the firstpillar extends from the top surface of the bulk substrate over the depthof the first trench, and the first pillar is connected to a portion ofthe bulk substrate beneath the first trench and the second trench. 4.The structure of claim 1 wherein the first waveguide core has a curvedshape that is wider than the first pillar.
 5. The structure of claim 1wherein the first pillar is narrower than the first portion of the firstdielectric layer, and the first pillar is narrower than the secondportion of the first dielectric layer.
 6. The structure of claim 1wherein the first waveguide core has a curved shape, and furthercomprising: a photodetector comprising a conformal layer positioned overthe curved shape of the first waveguide core.
 7. The structure of claim6 wherein the conformal layer comprises germanium.
 8. The structure ofclaim 1 further comprising: a heater positioned over the top surface ofthe bulk substrate; and a conductive layer coupling the heater to thefirst waveguide core.
 9. The structure of claim 8 wherein the bulksubstrate includes a third trench and a second pillar of thesemiconductor material laterally positioned between the second trenchand the third trench, the first dielectric layer has a third portion inthe third trench, and further comprising: a second waveguide corecoupled to the second pillar at the top surface of the bulk substrate,wherein the first waveguide core and the second waveguide core arerespective arms of a Mach-Zehnder interferometer modulator.
 10. Thestructure of claim 8 wherein the heater is a silicided resistor formedadjacent to the first waveguide core.
 11. The structure of claim 1further comprising: a transistor in a different region of the bulksubstrate than the first waveguide core.
 12. The structure of claim 1wherein the first waveguide core includes a p-type region, an n-typeregion, and a p-n junction at an interface between the p-type region andthe n-type region.
 13. The structure of claim 1 further comprising: aspacer in the first trench and on the first pillar; and a seconddielectric layer beneath the spacer, the second dielectric layerpositioned between the first portion of the first dielectric layer andthe first pillar.
 14. The structure of claim 13 wherein the spacer andthe second dielectric layer have a summed height relative to the topsurface of the bulk substrate, the first trench has a depth relative tothe top surface of the bulk substrate, and the depth of the first trenchis substantially equal to the summed height of the spacer and the seconddielectric layer.
 15. The structure of claim 1 wherein the first pillarhas a width ranging from about fifty nanometers to about one micron, andthe first pillar has an aspect ratio that is less than or equal tosixty.
 16. A method comprising: forming a first trench and a secondtrench in a substrate comprised of a semiconductor material, wherein apillar of the semiconductor material laterally positioned between thefirst trench and the second trench; forming a spacer in the first trenchand on the pillar; forming a second dielectric layer beneath the spacer;forming a first dielectric layer having a first portion in the firsttrench and a second portion in the second trench; and forming awaveguide core coupled to the pillar at a top surface of the substrate,wherein the second dielectric layer is positioned between the firstportion of the first dielectric layer and the pillar.
 17. The method ofclaim 16 further comprising: forming a heater positioned over the topsurface of the substrate; and forming a conductive layer coupling theheater to the waveguide core.
 18. The method of claim 16 wherein thewaveguide core has a curved shape, and further comprising: forming aphotodetector conformally positioned over the curved shape of thewaveguide core.
 19. The method of claim 16 wherein the waveguide core isformed by an epitaxial growth process, and further comprising: forming ap-type region and an n-type region in the waveguide core, wherein a p-njunction is defined at an interface between the p-type region and then-type region.
 20. The structure of claim 1 wherein the first pillar,the first trench, and the second trench extend to equal depths in thebulk substrate relative to the top surface of the bulk substrate.